The present invention relates to improved methods and apparatus for use in the drilling of subsea boreholes, particularly for the recovery of hydrocarbon products from subsea geological formations.
This invention is most directly applicable to the solution of particular problems encountered in the drilling of boreholes in extremely deep water. An example of such operations is the drilling of boreholes in the Gulf of Mexico on the downslope of the continental shelf, in water depths of the order of 6000 to 7000 feet (1830 to 2130 meters), where the hydrocarbon-bearing formation may be a further 10,000 feet (3050 meters) beneath the seabed.
At such depths, the seabed often includes a top layer, usually of the order of 300 to 400 feet (90 to 120 meters) deep, of unconsolidated, mud-like material, followed by a layer of unconsolidated sedimentary rock, before consolidated geologic formation is reached.
Running the borehole casing in deep water seabed conditions of this type is problematic by reason of the unconsolidated nature of the top seabed layers. The unconsolidated sedimentary formations referred to above often contain large, sealed volumes of over-pressured seawater. When a borehole penetrates a region of formation containing such water, the over-pressure causes water to flow out of the formation and into the borehole (xe2x80x9cshallow salt water flowxe2x80x9d). Such water flow may continue for long periods of time before the pressure in the formation equalises with the xe2x80x9cambientxe2x80x9d pressure, and can be likened to the flow of an underwater river. The volume and rate of flow are such that the borehole will normally be completely destroyed or else damaged to the extent that it has to be abandoned.
The invention relates to the particular problem of forming and casing the initial sections of a subsea borehole through layers of unconsolidated formation, where the depth of the unconsolidated formations is small compared to the depth of water in which the operation is to be performed. In order to drill a borehole to the hydrocarbon-bearing formations, it is necessary first of all to drill and stabilise initial borehole sections through the unconsolidated layers.
In the context of drilling operations of this type, it is well known to establish a first borehole section through the first layer of mud-like material using fluid jetting techniques, with the required casing being lowered closely behind the jetting tool so as to stabilise the first borehole section as it is formed. This in itself is a simple form of xe2x80x9ccasing while drillingxe2x80x9d, but does not utilise a rotating drill-bit. The depth of the water compared with the required length of casing (typically of the order of 300 feet to 400 feet (90 to 120 meters) is such that the complete casing string can be pre-assembled and hung-off from the drilling vessel with the jetting tool string extending through the casing, before lowering the casing and jetting string to the seabed.
Having established the first section of borehole, typically lined with 36 inch casing, the second section is drilled through the second layer, containing zones of overpressured water, typically using a 24 inch (60.96 cm) bit, and into the underlying formation, to accommodate 20 inch casing. This second borehole section may be of the order of 3000 feet (915 meters) in depth. Using conventional methods, the second borehole would be drilled in its entirety before withdrawing the drill string and running the casing string. If the borehole penetrates a zone of overpressured water, the resulting xe2x80x9cshallow water flowxe2x80x9d from the overpressured zone into the borehole will destroy or render useless the borehole before the casing can be run. This problem has been addressed in the past by pumping high density drilling fluid through the drill string, so as to fill the borehole with fluid at a higher pressure than the overpressured water in the surrounding formation However, this is extremely expensive, because the drilling fluid cannot be recirculated, and environmentally undesirable, because the drilling fluid is allowed to escape into the subsea environment.
One object of the present invention is therefore to mitigate the above problems by making use of the concept of casing-while-drilling in order to produce a stable, cased borehole through such unconsolidated sedimentary formations containing over-pressured seawater.
A number of additional problems also need to be addressed in order for casing while drilling to be applied in practice. These include the need to control fluid flow paths through the drill and casing strings and the need for blow-out-prevention measures. The most obvious solution to these problems is the use of over-size risers and blow-out-preventers (BOPs) as used in conventional, smaller diameter drilling. However, this is impractical for the large diameter borehole and casing sections with which the present invention is concerned. Cementing a 20 inch casing run so as to obtain a good cement bond in the zones subject to shallow water flow also presents practical difficulties.
Yet further, on certain occasions where the formation is exceptionally weak, the mere running of casing while drilling does not guarantee that the flow of over pressurised water will be controlled. It has been known for such unconsolidated layers to be so soft that the over pressurised water flows up the annulus outside the second section casing and thereafter breaks down the formation outside the 36 inch casing creating a damaging flow path on the outside of the top casing. An example of such an occurrence in recent times may be found in the URSA oil field where the drill template subsided below the mud-line.
In accordance with a first aspect of the present invention, there is provided a method of forming a subsea borehole from a drilling vessel in a body of water of known depth, comprising securing a casing string to a drill string and running said strings, thereby excavating a section of the borehole in a typically unconsolidated formation while isolating the annulus between the casing string and the borehole section from fluid circulation and creating a hydrostatic pressure in the annulus adapted to balance any over pressurised water in the formation.
The method may further comprise the prior steps of:
forming a first borehole section lined with a first casing having a first diameter;
making up the drill string and casing string with a second casing having a second diameter which is less than said first diameter, said casing string having an overall length less than the depth of the body of water, and hanging said casing string off from the drilling vessel;
running said drill string through the interior of said casing string, securing said casing string to said drill string and disconnecting said casing string from said drilling vessel; and
running said casing string and drill string together into the first borehole section.
Preferably the annulus is isolated from fluid circulation by means of sealing means provided on the outside of the casing string.
Sufficient hydrostatic pressure may be created in the annulus by pumping a suitable gel into the annulus above the sealing means. The gel might typically comprise of a mixed metal hydroxide or mixed metal silicate base. Preferably, the gel is pumped into the annulus during excavation of the said borehole section. Preferably, the first borehole section is formed by means of fluid jetting tool, said first casing being run simultaneously with said fluid jetting tool.
Preferably the said drill string is made up with a drill bit at its lowermost end and with a centraliser assembly incorporated therein above said drill bit.
Preferably also, said casing string is made up with at least a first port collar incorporated therein at a distance from the uppermost end of the casing string which is greater than the length of said first casing.
Preferably, said annular sealing means comprises at least one cup seal element. Most preferably, said sealing means comprises a plurality of cup seal elements spaced along the length of the casing string.
In accordance with a second aspect of the present invention there is provided a casing string having a plurality of sealing means spaced on the outer surface thereof, and further having attached thereto a feed line for the supply of gel into the annulus between the casing string and the second borehole section when the string is run.
Preferably, the casing string is adapted for attachment to a drill string.